Titanium alloy bar and method for manufacturing the same

ABSTRACT

The invention relates to an α+β type titanium alloy bar consisting essentially of 4 to 5% Al, 2.5 to 3.5% V, 1.5 to 2.5% Fe, 1.5 to 2.5% Mo, by mass, and balance of Ti, and having 10 to 90% of volume fraction of primary α phase, 10 μm or less of average grain size of the primary α phase, and 4 or less of aspect ratio of the grain of the primary α phase on the cross sectional plane parallel in the rolling direction of the bar. The α+β type titanium alloy bar has excellent ductility, fatigue characteristics and formability.

[0001] This application is a continuation application of InternationalApplication PCT/JP02/01710 (not published in English) filed Feb. 26,2002.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a titanium alloy bar havingexcellent ductility, fatigue characteristics and formability,particularly to an α+β type titanium alloy bar, and to a method formanufacturing thereof.

[0004] 2.Description of Related Arts

[0005] Owing to high strength, light weight and excellent corrosionresistance, titanium alloys are used as structural materials in thefields such as chemical plants, power generators, aircrafts and thelike. Among them, an α+β type titanium alloy occupies a large percentageof use because of its high strength and relatively good formability.

[0006] Products made of titanium alloys have various shapes such assheet, plate, bar and so on. The bar may be used as it is, or may beforged or formed in complex shapes such as a threaded fastener.Accordingly, the bar is requested to have excellent formability as wellas superior ductility and fatigue characteristics.

[0007]FIG. 1 shows a typical manufacturing method of bar.

[0008] An ingot prepared by melting is forged to a billet as a basematerial for hot rolling. As shown in FIG. 2A and FIG. 2B, the billet ishot rolled to a bar after reheated in a reheating furnace using areverse rolling mill or tandem rolling mills. If necessary, the billetis intermediately reheated during hot rolling to compensate thetemperature needed for subsequent hot rolling.

[0009] As for a titanium alloy bar, particularly as for an α+β typetitanium alloy bar, however, the temperature of billet increases duringhot rolling owing to the adiabatic heat, which disturbs stable hotrolling and manufacturing of a titanium alloy bar having excellentductility, fatigue characteristics and formability. For example, if thetemperature of billet increases to β transus or above, the finally hotrolled bar has β microstructure consisting mainly of acicular α phase,thus failing in attaining superior ductility and fatiguecharacteristics. In addition, even as for a Ti-6Al-4V alloy having highβ transus, the increase in temperature during hot rolling owing to theadiabatic heat enhances grain growth, although the temperature duringhot rolling hardly exceeds β transus, thus failing in attainingexcellent ductility, fatigue characteristics and formability.

[0010] To solve the problem of temperature increase during hot rollingcaused by the adiabatic heat, JP-A-59-82101, (the term “JP-A” referredherein signifies the “unexamined Japanese patent publication”),discloses a rolling method in which cross sectional area reduction rateof billet is specified to 40% or less per rolling pass in α region or inα+β region. JP-A-58-25465 discloses a method in which billet is watercooled during hot rolling to suppress the temperature rise caused by theadiabatic heat. Furthermore, Article 1 “Hot Bar Rolling of Ti-6Al-4V ina Continuous Mill (Titanium '92 Science and Technology)” describes thathot rolling speed is reduced to the lower limit of keeping performanceof mill in order to suppress the adiabatic heat.

[0011] The methods disclosed in JP-A-59-82101 and JP-A-58-25465,however, cannot produce a titanium alloy bar that simultaneously hasexcellent ductility, fatigue characteristics and formability.

[0012] Even if cross sectional area reduction rate per rolling is 40% orless according to the method of JP-A-59-82102, it is not sufficient tosuppress the adiabatic heat for some kinds of titanium alloys. Themethod of JP-A-58-25465 also causes characteristics deterioration byhydrogen absorption caused by water cooling, and difficulty in accuratetemperature control because of deformation resulted from rapid cooling.

[0013] The method described in Article 1 deals with a Ti-6Al-4V alloy.As described below, the method is not necessarily applicable to alloyswhich generate large adiabatic heat and therefor should be hot rolled inlow temperature region, resulting in poor ductility, fatiguecharacteristics and formability.

[0014]FIG. 3 shows a relationship between temperature and rolling timeduring hot rolling for Ti-6Al-4V alloy and Ti-4.5Al-3V-2Fe-2Mo alloy.

[0015] The heating temperature was 950° C. for the Ti-6Al-4V alloy, and850° C. for the Ti-4.5Al-3V-2Fe-2Mo alloy. The Ti-4.5Al-3V-2Fe-2Mo alloyhas lower β transus than that of the Ti-6Al-4V alloy by 100° C. so thatthe heating temperature was reduced by the difference, thus selecting850° C. as the heating temperature thereof. The rolling was conductedusing a reverse rolling mill and tandem rolling mills, while selectingthe same conditions of rolling speed, reduction rate and pass scheduleto both alloys. The rolling speed of reverse rolling mill was 2.7 m/sec,and the rolling speed of tandem rolling mills was 2.25 m/sec at thefinal rolling pass where the rolling speed becomes the maximum for bothalloys. The rolling speeds are lower than the rolling speed of Article 1(6 m/sec). The cross sectional area reduction rate was selected tomaximum 26% for both alloys.

[0016] For the case of the Ti-6Al-4V alloy, the rolling was conducted ata sufficiently lower temperature than 1000° C. which is the β transus ofthe alloy, thus giving favorable structure. For the case of theTi-4.5Al-3V-2Fe-2Mo alloy, however, even if the heating temperature wasdecreased by the magnitude of low β transus, the low temperature rollingresulted in increased deformation resistance and in increased adiabaticheat, so the temperature increased to a temperature region exceeding theβ transus, thus failed to obtain favorable microstructure. As a result,excellent ductility, fatigue characteristics and formability were notobtained. The result suggests that rolling conditions such as rollingtemperature, reduction rate and time between rolling passes shall beconsidered, as well as the rolling speed.

SUMMARY OF THE INVENTION

[0017] An object of the present invention is to provide a high strengthtitanium alloy bar having excellent ductility, fatigue characteristicsand formability, and to provide a method of manufacturing thereof.

[0018] The object is attained by an α+β type titanium alloy barconsisting essentially of 4 to 5% Al, 2.5 to 3.5% V, 1.5 to 2.5% Fe, 1.5to 2.5% Mo, by mass, and balance of Ti, and having 10 to 90% of volumefraction of primary α phase, 10 μm or less of average grain size of theprimary α phase, and 4 or less of aspect ratio of the grain of theprimary α phase on the cross sectional plane parallel in the rollingdirection of the bar.

[0019] The α+β type titanium alloy bar can be manufactured by a methodcomprising the step of hot rolling an α+β type titanium alloy consistingessentially of 4 to 5% Al, 2.5 to 3.5% V, 1.5 to 2.5% Fe, 1.5 to 2.5%Mo, by mass, and balance of Ti, while keeping the surface temperaturethereof to β transus or below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 shows a typical method for manufacturing a bar.

[0021]FIG. 2 shows a process for hot rolling a bar.

[0022]FIG. 3 shows a relationship between temperature and rolling timeduring hot rolling for Ti-6Al-4V alloy and Ti-4.5Al-3V-2Fe-2Mo alloy.

[0023]FIG. 4 shows a relationship between average grain size of primaryα phase and total elongation measured by high temperature tensile test.

[0024]FIG. 5 shows a relationship between average grain size of primaryα phase and fatigue strength after 10⁸ cycles observed in fatigue test.

[0025]FIG. 6 shows temperature changes with time at surface and center.

[0026]FIG. 7 shows a relationship between cross sectional area andtemperature difference between surface and center.

DETAILED DESCRIPTION OF THE INVENTION

[0027] The inventors of the present invention studied the microstructureof α+β type titanium alloy bar to provide excellent ductility, fatiguecharacteristics and formability, and found the followings.

[0028] The α+β type titanium alloy consists of primary α phase andtransformed β phase. If, however, the alloy contains very large volumefraction of α phase that has HCP structure having little sliding system,or contains very large volume fraction of transformed β phase containingacicular α phase, formability and ductility deteriorate. Consequently,the volume fraction of primary α phase is specified to a range of from10 to 90%. If the volume fraction of α phase and of β phase is equal orclose to each other at reheating stage before hot rolling, theformability becomes better, so the volume fraction of primary α phase ispreferably between 50 and 80%.

[0029]FIG. 4 shows a relationship between average grain size of primaryα phase and total elongation measured by high temperature tensile test.

[0030] When the average grain size of primary α phase exceeds 10 μm, thetotal elongation measured by high temperature tensile test rapidlydecreases, and therefore the formability degrades.

[0031]FIG. 5 shows a relationship between average grain size of primaryα phase and fatigue strength after 10⁸ cycles observed in fatigue test.

[0032] If the average grain size of primary α phase exceeds 10 μm, thefatigue strength decreases. If the average grain size of primary α phasebecomes less than 6 μm, higher fatigue strength is attained.

[0033] Forging a bar induces rough surface on a free deforming plane notcontacting with a mold due to the shape of grains, or due to the aspectratio of the grains. Generally, the grains of bar tend to be elongatedin the rolling direction. Particularly for the case of upset forging,elongated grains appear on a side face of the bar that becomes a freedeforming plane. Therefore, it is necessary to avoid excessive increasein the aspect ratio during forging, more concretely to regulate theaspect ratio not exceeding 4 for the grains of the primary α phase on across section parallel in the rolling direction of the bar in order toprevent rough surface on the bar after forged.

[0034] Based on the above-described findings, a high strength titaniumalloy bar having excellent ductility, fatigue characteristics andformability is obtained when the volume fraction of the primary α phaseis between 10 and 90%, preferably between 50 and 80%, the average grainsize in the primary α phase is 10 μm or less, preferably 6 μm or less,and further the aspect ratio of grains in the primary α phase is 4 orless.

[0035] The α+β type titanium alloy bar having above-describedmicrostructure should consist essentially of 4 to 5% Al, 2.5 to 3.5% V,1.5 to 2.5% Fe, 1.5 to 2.5% Mo, by mass, and balance of Ti. The reasonsto limit the content of individual elements are described below.

[0036] Al

[0037] Aluminum is an essential element to stabilize the α phase and tocontribute to the strength increase. If the Al content is below 4%, highstrength cannot fully be attained. If the Al content exceeds 5%,ductility degrades.

[0038] V

[0039] Vanadium is an element to stabilize the β phase and to contributeto the strength increase. If the V content is below 2.5%, high strengthcannot fully be attained, and β phase becomes unstable. If the V contentexceeds 3.5%, range of workable temperature becomes narrow caused by thelowered β transus, and cost increases.

[0040] Mo

[0041] Molybdenum is an element to stabilize the β phase and tocontribute to the strength increase. If the Mo content is below 1.5%,high strength cannot fully be attained, and β phase becomes unstable. Ifthe Mo content exceeds 2.5%, range of workable temperature becomesnarrow-caused by the lowered β transus, and cost increases.

[0042] Fe

[0043] Iron is an element to stabilize the β phase and to contribute tothe strength increase. Iron rapidly diffuses to improve formability. If,however, the Fe content is below 1.5%, high strength cannot fully beattained, and the β phase becomes unstable, which results in failing toattain excellent formability. If the Fe content exceeds 2.5%, range ofworkable temperature becomes narrow caused by the lowered β transus, anddegradation in characteristics is induced by segregation.

[0044] The α+β type titanium alloy bar according to the presentinvention may be manufactured by hot rolling an α+β type titanium alloyhaving above-described composition while adjusting the conditions ofheating temperature, rolling temperature range, reduction rate, rollingspeed, time between passes, and other variables to suppress thetemperature rise caused by the adiabatic g heat, namely to keep thesurface temperature of the alloy not exceeding the β transus. Forexample, the method comprises the steps of: heating an α+β type titaniumalloy having β transus of Tβ ° C. so that the surface temperature rangesbetween (Tβ−150) and Tβ ° C.; and hot rolling the heated α+β typetitanium alloy so that the surface temperature thereof during hotrolling is between (Tβ−300) and (Tβ−50) ° C., and so that the finishsurface temperature thereof is between (Tβ−300) and (Tβ−100) ° C.

[0045] The reason of heating the surface before hot rolling in the rangeof from (Tβ−150) to Tβ ° C. is the following. If the surface temperaturebefore hot rolling is below (Tβ−150) ° C., the decrease in temperatureduring the final rolling stage becomes significant to increase cracksusceptibility and deformation resistance. And, if the surfacetemperature before hot rolling exceeds Tβ ° C., the microstructure ofthe bar becomes β microstructure consisting mainly of acicular α phase,which deteriorates ductility and formability. The reason of limiting thesurface temperature during hot rolling to the range of from (Tβ−300) to(Tβ−50) ° C. is the following. If the surface temperature during hotrolling is below (Tβ−300) ° C., the hot formability deteriorates toinduce problems such as cracking. And, if the surface temperature duringhot rolling exceeds (Tβ−50) ° C., the temperature rise caused by theadiabatic heat induces coarse grains and formation of acicular phase.The reason of limiting the finish surface temperature immediately afterthe final rolling pass to the range of from (Tβ−300) and (Tβ−100) ° C.is the following. If the finish temperature thereof is below (Tβ−300) °C., the crack susceptibility and the deformation resistance increase.And, if the finish temperature thereof exceeds (Tβ−100) ° C., grainsbecome coarse.

[0046] The hot rolling is conducted by plurality of rolling passes. Toprevent temperature rise caused by the adiabatic heat, it is preferableto keep the reduction rate not more than 40% per rolling pass.

[0047] When the hot rolling is conducted by a reverse rolling mill, itis preferable to limit the rolling speed not more than 6 m/sec toprevent the temperature rise caused by the adiabatic heat. When the hotrolling is conducted by tandem rolling mills, it is preferable to limitthe rolling speed not more than 1.5 m/sec.

[0048] Since the alloy is cooled from surface after each rolling pass,the surface of the alloy receives temperature drop to some extent beforeentering succeeding pass even if a temperature rise exists caused by theadiabatic heat. As shown in FIG. 6, however, if the alloy has a largediameter (for the case of 106 mm in diameter), the temperature drop atcenter section of the alloy is small so that a large temperaturedifference appears between the surface and the center of the alloy. Whenthe temperature drop at the center is small, the alloy is subjected tosucceeding rolling pass before lowering the temperature of the center,which further increases the temperature owing to the adiabatic heat. Ifthe phenomenon sustains, the center is hot rolled at higher temperaturethan the initial temperature. Consequently, the center of alloy havinglarge diameter is required to be cooled with sufficient time betweenrolling passes.

[0049] To this point, the inventors of the present invention made adetailed study on the temperature difference between the surface and thecenter, and derived the finding described below. As shown in FIG. 7, thetemperature difference significantly increases at or above 3500 mm² ofcross sectional area of alloy normal to the rolling direction thereof.When an alloy having large cross sectional area is hot rolled to S mm²of the cross sectional area, securing the time before enteringsucceeding rolling at 0.167×S^(1/2) sec or more can make the temperaturedifference small and is favorable in manufacturing a bar havinghomogeneous characteristics.

[0050] According to the manufacturing method of the present invention,the hot rolling is carried out while keeping the surface temperature ofthe alloy to β transus or below, thus there is a possibility for thesurface temperature to decrease to a lower than the required rollingtemperature range during hot rolling depending on the time betweenrolling passes and on the diameter of alloy. In that case, reheating thealloy may be given using a high frequency heating unit or the like.

EXAMPLE 1

[0051] Materials having 125 square mm size were prepared by cutting eachof the base alloy A01 (having composition within the range of thepresent invention) and the base alloy A02 (having composition outsidethe range of the present invention), both of which are α+β type titaniumalloy having respective chemical compositions given in Table 1. Thematerials are hot rolled using a caliber rolling mill under respectiveconditions (B01 through B18) given in Table 2 to produce bars having 20mm and 50 mm in diameter, respectively. For the time between rollingpasses given in Table 2, ◯ denotes the time between rolling passes of0.167×S^(1/2) or more for all the rolling passes under each rollingcondition, and X denotes the time between rolling passes of less than0.167×S^(1/2). Table 3 through Table 20 give cross sectional area S ofalloy, reduction rate, 0.167×S^(1/2), time between rolling passes,surface temperature, and rolling speed on each rolling pass under eachrolling condition. R in the table signifies a reverse rolling mill, andT signifies tandem rolling mills.

[0052] The produced bars were annealed at temperatures between 700 and720° C. Tensile test was conducted to determine yield strength (0.2%PS), tensile strength (UTS), elongation (El), and reduction of area(RA). In addition, the smooth fatigue test (under the condition of Kt=1)and the notch fatigue test (under the condition of Kt=3) were given todetermine fatigue strength.

[0053] Furthermore, optical microstructure examination was performed atthe center of the bar and at the position of quarter of diameter (¼ D)to determine grain size of primary α phase, volume fraction of thegrains, and aspect ratio of the grains on a cross section parallel inthe rolling direction.

[0054] The results are given in Table 21. The columns of themicrostructure in the table giving no grain size mean that the positionconsisted only of β microstructure consisting mainly of acicular α phaseand that the equiaxed primary α phase could not be observed.

[0055] When the surface heating temperature is below (Tβ−150) ° C., thesurface temperature of the alloy was excessively low, and the rollingload became excessive to fail in rolling. When the heating temperatureexceeds Tβ ° C., the surface temperature of the alloy became too higheven if the time between rolling passes was within the range of thepresent invention, which is seen under the rolling conditions of B02 andB11, so the surface temperature exceeded Tβ ° C. caused by the adiabaticheat to form β microstructure consisting mainly of acicular α phase atthe center of the bar, thus deteriorated ductility and fatiguecharacteristics.

[0056] When the finish surface temperature was below (Tβ−300) ° C., thetemperature of the alloy became too low, which deteriorated formabilityto generate cracks during hot rolling. When the finish surfacetemperature exceeded (Tβ−100) ° C., fine microstructure could not beattained, deteriorating ductility and fatigue characteristics as in thecases under the conditions of B04, B05, and B07.

[0057] When the surface temperature during hot rolling was below(Tβ−300) ° C., the surface temperature was too low, generating cracks.When the surface temperature exceeded (Tβ−50) ° C., the center and the ¼D had β microstructure consisting mainly of acicular α phase after hotrolling, deteriorating ductility and fatigue characteristics.

[0058] When the reduction rate per rolling pass exceeded 40%, theadiabatic heat was enhanced, and the temperature of the alloy exceededTβ ° C., and fine microstructure could not be attained.

[0059] In the case of the rolling condition B14 which applied a reverserolling mill and which selected the rolling speeds of higher than 6m/sec, or in the case of rolling condition B15 which applied tandemrolling mills and which selected the rolling speeds of higher than 1.5m/sec, the adiabatic heat became large, and the surface temperatureexceeded Tβ ° C., thus failed to attain fine microstructure.

[0060] When the time between rolling passes was outside the range of thepresent invention, the surface temperature increase caused by theadiabatic heat overrode the temperature decrease caused by air cooling,thus the surface temperature exceeded Tβ ° C., and fine microstructurecould not be attained.

[0061] With the bars using A01 which had the chemical composition withinthe range of the present invention and produced under the rollingconditions B01, B06, B08, B09, B16, B17, and B18, homogeneousmicrostructure of 10 μm or smaller grain size of primary α phase wasobserved, and they provided excellent ductility and fatiguecharacteristics. That is, further excellent ductility and fatiguecharacteristics could be attained giving 15% or larger elongation, 40%or larger reduction of area, 500 MPa or larger smooth fatigue strength,and 200 MPa of notch (Kt=3) fatigue strength. Furthermore, with the α+βtype titanium alloy bars having 50 to 80% of volume fraction of primaryα phase and 6 μm or less of average grain size of primary α phase,produced under the rolling conditions of B01, B06, B08, and B09, furtherexcellent ductility and fatigue characteristics could be attained giving20% or larger elongation, 50% or larger reduction of area, 550 MPa orlarger smooth fatigue strength, and 200 MPa of notch (Kt=3) fatiguestrength.

[0062] On the other hand, bars produced using A02 having chemicalcomposition outside the range of the present invention under the rollingconditions of B10 and B12 could not attain satisfactory ductility andfatigue characteristics because the grain size in the primary α phaseexceeded 10 μm, though the adiabatic heat was suppressed because therolling conditions were within the range of the present invention.

EXAMPLE 2

[0063] Cylindrical specimens having 8 mm in diameter and 12 mm in heightwere cut from the center section in radial direction of bars produced inExample 1 under the rolling conditions B01 through B18, respectively.The specimens were heated to 800° C. and were compressed to 70%. Afterthe compression, the occurrence of cracks and of rough surface on thesurface of each specimen was inspected to give evaluation of hot forgingproperty.

[0064] The results are shown in Table 21.

[0065] As for the bars produced under the rolling conditions of B01,B06, B08, B09, B16, B17, and B18 which were within the range of thepresent invention, no crack and rough surface appeared, and favorablehot forging property was obtained.

[0066] On the other hand, for the bars produced under the rollingconditions of B10 and B12 in which the grain size in the primary α phaseexceeded 10 μm, rough surface appeared, though no crack was generated.As for the bars having only α phase at center and ¼ D produced under therolling conditions of B02, B03, B04, B05, B07, B11, B14, and B15, bothcracks and rough surface appeared. Furthermore, for the bars producedunder the rolling condition B14 giving aspect ratios of more than 4 forthe grains in a cross section parallel in the rolling direction, thoughgiving the grain size in the primary α phase and the volume fractionwithin the range of the present invention, rough surface also appeared.TABLE 1 β Alloy Al V Fe Mo O C N H transus A01 4.7 3.1 2.1 1.9 0.1 0.0010.005 0.0017  900° C. A02 6.1 4.1 0.2 — 0.2 0.01 0.006 0.0016 1000° C.

[0067] TABLE 2 Rolling speed Final rolling Maximum in speed in RollingTotal reduction rough rolling finish rolling Finish Reheat- temp. FinishTime number rate (Reverse (Tandem Rolling diameter ing temp. range temp.between of per rolling rolling mill) rolling mills) condition Alloy (mm)(° C.) (° C.) (° C.) passes passes pass (%) (m/sec) (m/sec) Remarks B01A01 φ20 800 700-811 714 ◯ 17 25.8 2.7 1.125 E B02 A01 φ20 950 755-929765 ◯ 17 25.8 2.7 1.125 C B03 A01 φ20 890 754-911 764 ◯ 17 25.8 2.71.125 C B04 A01 φ20 850 818-930 919 ◯ 8 42.4 2.7 1.125 C B05 A01 φ20 800845-901 865 X 17 25.8 2.7 1.125 C B06 A01 φ50 800 711-804 731 ◯ 12 18.42.7 1.125 E B07 A01 φ50 830 864-909 874 X 12 18.4 2.7 1.125 C B08 A01φ20 800 670-812 690 ◯ 17 25.8 2.7 1.125 E B09 A01 φ20 820 721-829 726 ◯17 25.8 2.7 1.125 E B10 A02 φ20 900 791-887 806 ◯ 17 25.8 2.7 1.125 CB11 A02 φ20 1050  815-1024 825 ◯ 17 25.8 2.7 1.125 C B12 A02 φ50 900810-906 830 ◯ 12 18.4 2.7 1.125 C B13 A01 φ20 920 698-928 698 ◯ 17 25.82.7 1.125 C B14 A01 φ20 800 774-911 774 ◯ 17 25.8 10.8 1.125 C B15 A01φ20 800 719-910 864 ◯ 17 25.8 2.7 2.250 C B16 A01 φ50 830 764-845 766 ◯12 18.4 2.7 1.125 E B17 A01 φ20 830 757-842 777 ◯ 17 25.8 2.7 1.125 EB18 A01 φ20 865 772-850 772 ◯ 17 25.8 2.7 1.125 E

[0068] TABLE 3 Rolling condition: B01 Number of Cross sectionalReduction 0.167{square root}{square root over (S)} Time between Rollingspeed Temp. Rolling passes area (mm²) rate (*) (sec) passes (sec)(m/sec) (° C.) mill 15625 1 13000 16.8 19.0 25 2.7 790 R 2 11000 15.417.5 25 2.7 796 R 3 9500 13.6 16.3 25 2.7 801 R 4 8000 15.8 14.9 25 2.7803 R 5 6500 18.8 13.5 25 2.7 811 R 6 5200 20.0 12.0 25 2.7 801 R 7 415020.2 10.8 25 2.7 779 R 8 3300 20.5 9.6 25 2.7 761 R 9 2450 25.8 8.3 252.7 738 R 10 1850 24.5 7.2 25 2.7 719 R 11 1450 21.6 6.4 5 0.350 721 T12 1150 20.7 5.7 5 0.466 732 T 13 900 21.7 5.0 5 0.581 739 T 14 700 22.24.4 5 0.733 745 T 15 550 21.4 3.9 5 0.871 741 T 16 420 23.6 3.4 5 0.982730 T 17 320 23.8 1.125 714 T

[0069] TABLE 4 Rolling condition: B02 Number of Cross sectionalReduction 0.167{square root}{square root over (S)} Time between Rollingspeed Temp. Rolling passes area (mm²) rate (*) (sec) passes (sec)(m/sec) (° C.) mill 15625 1 13000 16.8 19.0 25 2.7 929 R 2 11000 15.417.5 25 2.7 925 R 3 9500 13.6 16.3 25 2.7 919 R 4 8000 15.8 14.9 25 2.7913 R 5 6500 18.8 13.5 25 2.7 911 R 6 5200 20.0 12.0 25 2.7 900 R 7 415020.2 10.8 25 2.7 891 R 8 3300 20.5 9.6 25 2.7 880 R 9 2450 25.8 8.3 252.7 868 R 10 1850 24.5 7.2 25 2.7 860 R 11 1450 21.6 6.4 5 0.350 852 T12 1150 20.7 5.7 5 0.466 839 T 13 900 21.7 5.0 5 0.581 829 T 14 700 22.24.4 5 0.733 822 T 15 550 21.4 3.9 5 0.871 803 T 16 420 23.6 3.4 5 0.982785 T 17 320 23.8 1.125 765 T

[0070] TABLE 5 Rolling condition: B03 Number of Cross sectionalReduction 0.167{square root}{square root over (S)} Time between Rollingspeed Temp. Rolling passes area (mm²) rate (*) (sec) passes (sec)(m/sec) (° C.) mill 15625 1 13000 16.8 19.0 25 2.7 890 R 2 11000 15.417.5 25 2.7 894 R 3 9500 13.6 16.3 25 2.7 899 R 4 8000 15.8 14.9 25 2.7906 R 5 6500 18.8 13.5 25 2.7 911 R 6 5200 20.0 12.0 25 2.7 902 R 7 415020.2 10.8 25 2.7 889 R 8 3300 20.5 9.6 25 2.7 881 R 9 2450 25.8 8.3 252.7 867 R 10 1850 24.5 7.2 25 2.7 860 R 11 1450 21.6 6.4 5 0.350 852 T12 1150 20.7 5.7 5 0.466 839 T 13 900 21.7 5.0 5 0.581 830 T 14 700 22.24.4 5 0.733 820 T 15 550 21.4 3.9 5 0.871 803 T 16 420 23.6 3.4 5 0.982784 T 17 320 23.8 1.125 764 T

[0071] TABLE 6 Rolling condition: B04 Number of Cross sectionalReduction 0.167{square root}{square root over (S)} Time between Rollingspeed Temp. Rolling passes area (mm²) rate (*) (sec) passes (sec)(m/sec) (° C.) mill 15625 1 9300 40.5 19.0 25 2.7 849 R 2 5500 40.9 17.525 2.7 865 R 3 3300 40.0 16.3 25 2.7 879 R 4 1900 42.4 14.9 25 2.7 896 R5 1100 42.1 13.5 25 2.7 912 R 6 660 40.0 12.0 25 2.7 921 R 7 400 39.410.8 25 2.7 930 R 8 320 20.0 2.7 919 R

[0072] TABLE 7 Rolling condition: B05 Number of Cross sectionalReduction 0.167{square root}{square root over (S)} Time between Rollingspeed Temp. Rolling passes area (mm²) rate (*) (sec) passes (sec)(m/sec) (° C.) mill 15625 1 13000 16.8 19.0 10 2.7 791 R 2 11000 15.417.5 10 2.7 805 R 3 9500 13.6 16.3 10 2.7 819 R 4 8000 15.8 14.9 10 2.7836 R 5 6500 18.8 13.5 10 2.7 850 R 6 5200 20.0 12.0 10 2.7 865 R 7 415020.2 10.8 10 2.7 871 R 8 3300 20.5 9.6 10 2.7 875 R 9 2450 25.8 8.3 102.7 879 R 10 1850 24.5 7.2 10 2.7 884 R 11 1450 21.6 6.4 5 0.350 901 T12 1150 20.7 5.7 5 0.466 899 T 13 900 21.7 5.0 5 0.581 895 T 14 700 22.24.4 5 0.733 895 T 15 550 21.4 3.9 5 0.871 883 T 16 420 23.6 3.4 5 0.982875 T 17 320 23.8 1.125 860 T

[0073] TABLE 8 Rolling condition: B06 Number of Cross sectionalReduction 0.167{square root}{square root over (S)} Time between Rollingspeed Temp. Rolling passes area (mm²) rate (*) (sec) passes (sec)(m/sec) (° C.) mill 15625 1 13000 16.8 19.0 25 2.7 791 R 2 11000 15.417.5 25 2.7 796 R 3 9500 13.6 16.3 25 2.7 801 R 4 8000 15.8 14.9 25 2.7804 R 5 6700 16.3 13.7 25 2.7 806 R 6 6000 10.5 12.9 25 2.7 784 R 7 520013.3 12.0 25 2.7 764 R 8 4650 10.6 11.4 25 2.7 746 R 9 3800 18.3 10.3 252.7 733 R 10 3100 18.4 9.3 5 0.622 733 T 11 2600 16.1 8.5 5 0.837 734 T12 2210 15.0 1.125 731 T

[0074] TABLE 9 Rolling condition: B07 Number of Cross sectionalReduction 0.167{square root}{square root over (S)} Time between Rollingspeed Temp. Rolling passes area (mm²) rate (*) (sec) passes (sec)(m/sec) (° C.) mill 15625 1 13000 16.8 19.0 10 2.7 819 R 2 11000 15.417.5 10 2.7 836 R 3 9500 13.6 16.3 10 2.7 849 R 4 8000 15.8 14.9 10 2.7873 R 5 6700 16.3 13.7 10 2.7 879 R 6 6000 10.5 12.9 10 2.7 896 R 7 520013.3 12.0 10 2.7 901 R 8 4650 10.6 11.4 10 2.7 904 R 9 3800 18.3 10.3 52.7 909 R 10 3100 18.4 9.3 5 0.622 902 T 11 2600 16.1 8.5 5 0.837 883 T12 2210 15.0 1.125 874 T

[0075] TABLE 10 Rolling condition: B08 Number of Cross sectionalReduction 0.167{square root}{square root over (S)} Time between Rollingspeed Temp. Rolling passes area (mm²) rate (*) (sec) passes (sec)(m/sec) (° C.) mill 15625 1 13000 16.8 19.0 25 2.7 790 R 2 11000 15.417.5 25 2.7 795 R 3 9500 13.6 16.3 25 2.7 799 R 4 8000 15.8 14.9 25 2.7804 R 5 6500 18.8 13.5 25 2.7 812 R 6 5200 20.0 12.0 25 2.7 800 R 7 415020.2 10.8 25 2.7 780 R 8 3300 20.5 9.6 25 2.7 759 R 9 2450 25.8 8.3 252.7 741 R 10 1850 24.5 7.2 25 2.7 720 R 11 1450 21.6 6.4 10 0.350 719 T12 1150 20.7 5.7 10 0.466 724 T 13 900 21.7 5.0 10 0.581 730 T 14 70022.2 4.4 10 0.733 729 T 15 550 21.4 3.9 10 0.871 721 T 16 420 23.6 3.410 0.982 705 T 17 320 23.8 1.125 690 T

[0076] TABLE 11 Rolling condition: B09 Number of Cross sectionalReduction {square root}{square root over (S)} Time between Rolling speedTemp. Rolling passes area (mm²) rate (*) (sec) passes (sec) (m/sec) (°C.) mill 15625 1 13000 16.8 19.0 25 2.7 810 R 2 11000 15.4 17.5 25 2.7816 R 3 9500 13.6 16.3 25 2.7 821 R 4 8000 15.8 14.9 25 2.7 824 R 5 650018.8 13.5 25 2.7 829 R 6 5200 20.0 12.0 25 2.7 821 R 7 4150 20.2 10.8 252.7 800 R 8 3300 20.5 9.6 25 2.7 779 R 9 2450 25.8 8.3 25 2.7 761 R 101850 24.5 7.2 25 2.7 749 R 11 1450 21.6 6.4 5 0.350 741 T 12 1150 20.75.7 5 0.466 751 T 13 900 21.7 5.0 5 0.581 760 T 14 700 22.2 4.4 5 0.733766 T 15 550 21.4 3.9 5 0.871 761 T 16 420 23.6 3.4 5 0.982 751 T 17 32023.8 1.125 726 T

[0077] TABLE 12 Rolling condition: B10 Number of Cross sectionalReduction {square root}{square root over (S)} Time between Rolling speedTemp. Rolling passes area (mm²) rate (*) (sec) passes (sec) (m/sec) (°C.) mill 15625 1 13000 16.8 19.0 25 2.7 886 R 2 11000 15.4 17.5 25 2.7884 R 3 9500 13.6 16.3 25 2.7 884 R 4 8000 15.8 14.9 25 2.7 887 R 5 650018.8 13.5 25 2.7 885 R 6 5200 20.0 12.0 25 2.7 859 R 7 4150 20.2 10.8 252.7 841 R 8 3300 20.5 9.6 25 2.7 820 R 9 2450 25.8 8.3 25 2.7 800 R 101850 24.5 7.2 25 2.7 791 R 11 1450 21.6 6.4 5 0.350 801 T 12 1150 20.75.7 5 0.466 810 T 13 900 21.7 5.0 5 0.581 830 T 14 700 22.2 4.4 5 0.733836 T 15 550 21.4 3.9 5 0.871 829 T 16 420 23.6 3.4 5 0.982 821 T 17 32023.8 1.125 806 T

[0078] TABLE 13 Rolling condition: B11 Number of Cross sectionalReduction {square root}{square root over (S)} Time between Rolling speedTemp. Rolling passes area (mm²) rate (*) (sec) passes (sec) (m/sec) (°C.) mill 15625 1 13000 16.8 19.0 25 2.7 1024 R 2 11000 15.4 17.5 25 2.71015 R 3 9500 13.6 16.3 25 2.7 1003 R 4 8000 15.8 14.9 25 2.7 996 R 56500 18.8 13.5 25 2.7 985 R 6 5200 20.0 12.0 25 2.7 969 R 7 4150 20.210.8 25 2.7 961 R 8 3300 20.5 9.6 25 2.7 949 R 9 2450 25.8 8.3 25 2.7930 R 10 1850 24.5 7.2 25 2.7 921 R 11 1450 21.6 6.4 5 0.350 911 T 121150 20.7 5.7 5 0.466 901 T 13 900 21.7 5.0 5 0.581 891 T 14 700 22.24.4 5 0.733 881 T 15 550 21.4 3.9 5 0.871 864 T 16 420 23.6 3.4 5 0.982845 T 17 320 23.8 1.125 825 T

[0079] TABLE 14 Rolling condition: B12 Number of Cross sectionalReduction {square root}{square root over (S)} Time between Rolling speedTemp. Rolling passes area (mm²) rate (*) (sec) passes (sec) (m/sec) (°C.) mill 15625 1 13000 16.8 19.0 25 2.7 891 R 2 11000 15.4 17.5 25 2.7895 R 3 9500 13.6 16.3 25 2.7 899 R 4 8000 15.8 14.9 25 2.7 905 R 5 670016.3 13.7 25 2.7 906 R 6 6000 10.5 12.9 25 2.7 886 R 7 5200 13.3 12.0 252.7 865 R 8 4650 10.6 11.4 25 2.7 845 R 9 3800 18.3 10.3 25 2.7 836 R 103100 18.4 9.3 5 0.622 835 T 11 2600 16.1 8.5 5 0.837 834 T 12 2210 15.01.125 830 T

[0080] TABLE 15 Rolling condition: B13 Number of Cross sectionalReduction {square root}{square root over (S)} Time between Rolling speedTemp. Rolling passes area (mm²) rate (*) (sec) passes (sec) (m/sec) (°C.) mill 15625 1 13000 16.8 19.0 25 2.7 929 R 2 11000 15.4 17.5 25 2.7925 R 3 9500 13.6 16.3 25 2.7 919 R 4 8000 15.8 14.9 25 2.7 913 R 5 650018.8 13.5 25 2.7 911 R 6 5200 20.0 12.0 25 2.7 900 R 7 4150 20.2 10.8 252.7 891 R 8 3300 20.5 9.6 25 2.7 880 R 9 2450 25.8 8.3 25 2.7 868 R 101850 24.5 7.2 25 2.7 850 R 11 1450 21.6 6.4 10 0.350 832 T 12 1150 20.75.7 10 0.466 804 T 13 900 21.7 5.0 10 0.581 777 T 14 700 22.2 4.4 100.733 749 T 15 550 21.4 3.9 10 0.871 728 T 16 420 23.6 3.4 10 0.982 713T 17 320 23.8 1.125 698 T

[0081] TABLE 16 Rolling condition: B14 Number of Cross sectionalReduction {square root}{square root over (S)} Time between Rolling speedTemp. Rolling passes area (mm²) rate (*) (sec) passes (sec) (m/sec) (°C.) mill 15625 1 13000 16.8 19.0 25 10.8 810 R 2 11000 15.4 17.5 25 10.8836 R 3 9500 13.6 16.3 25 10.8 861 R 4 8000 15.8 14.9 25 10.8 883 R 56500 18.8 13.5 25 10.8 911 R 6 5200 20.0 12.0 25 10.8 901 R 7 4150 20.210.8 25 10.8 869 R 8 3300 20.5 9.6 25 1.8 841 R 9 2450 25.8 8.3 25 10.8808 R 10 1850 24.5 7.2 25 10.8 779 R 11 1450 21.6 6.4 10 0.350 781 T 121150 20.7 5.7 10 0.466 792 T 13 900 21.7 5.0 10 0.581 799 T 14 700 22.24.4 10 0.733 805 T 15 550 21.4 3.9 10 0.871 801 T 16 420 23.6 3.4 100.982 790 T 17 320 23.8 1.125 774 T

[0082] TABLE 17 Rolling condition: B15 Number of Cross sectionalReduction {square root}{square root over (S)} Time between Rolling speedTemp. Rolling passes area (mm²) rate (*) (sec) passes (sec) (m/sec) (°C.) mill 15625 1 13000 16.8 19.0 25 2.7 790 R 2 11000 15.4 17.5 25 2.7796 R 3 9500 13.6 16.3 25 2.7 801 R 4 8000 15.8 14.9 25 2.7 803 R 5 650018.8 13.5 25 2.7 811 R 6 5200 20.0 12.0 25 2.7 801 R 7 4150 20.2 10.8 252.7 779 R 8 3300 20.5 9.6 25 2.7 761 R 9 2450 25.8 8.3 25 2.7 738 R 101850 24.5 7.2 25 2.7 719 R 11 1450 21.6 6.4 5 0.700 751 T 12 1150 20.75.7 5 0.932 782 T 13 900 21.7 5.0 5 1.162 829 T 14 700 22.2 4.4 5 1.466865 T 15 550 21.4 3.9 5 1.742 891 T 16 420 23.6 3.4 5 1.964 910 T 17 32023.8 1.500 864 T

[0083] TABLE 18 Rolling condition: B16 Number of Cross sectionalReduction {square root}{square root over (S)} Time between Rolling speedTemp. Rolling passes area (mm²) rate (*) (sec) passes (sec) (m/sec) (°C.) mill 15625 1 13000 16.8 19.0 25 2.7 821 R 2 11000 15.4 17.5 25 2.7817 R 3 9500 13.6 16.3 25 2.7 834 R 4 8000 15.8 14.9 25 2.7 838 R 5 670016.3 13.7 25 2.7 845 R 6 6000 10.5 12.9 25 2.7 824 R 7 5200 13.3 12.0 252.7 794 R 8 4650 10.6 11.4 25 2.7 776 R 9 3800 18.3 10.3 25 2.7 767 R 103100 18.4 9.3 5 0.622 764 T 11 2600 16.1 8.5 5 0.837 769 T 12 2210 15.01.125 766 T

[0084] TABLE 19 Rolling condition: B17 Number of Cross sectionalReduction 0.167{square root}{square root over (S)} Time between Rollingspeed Temp. Rolling passes area (mm²) rate (*) (sec) passes (sec)(m/sec) (° C.) mill 15625 1 13000 16.8 19.0 25 2.7 822 R 2 11000 15.417.5 25 2.7 825 R 3 9500 13.6 16.3 25 2.7 833 R 4 8000 15.8 14.9 25 2.7834 R 5 6500 18.8 13.5 25 2.7 842 R 6 5200 20.0 12.0 25 2.7 830 R 7 415020.2 10.8 25 2.7 809 R 8 3300 20.5 9.6 25 2.7 790 R 9 2450 25.8 8.3 252.7 765 R 10 1850 24.5 7.2 25 2.7 757 R 11 1450 21.6 6.4 5 0.350 759 T12 1150 20.7 5.7 5 0.466 772 T 13 900 21.7 5.0 5 0.581 771 T 14 700 22.24.4 5 0.733 774 T 15 550 21.4 3.9 5 0.871 771 T 16 420 23.6 3.4 5 0.982779 T 17 320 23.8 1.125 777 T

[0085] TABLE 18 Rolling condition: B18 Number of Cross sectionalReduction {square root}{square root over (S)} Time between Rolling speedTemp. Rolling passes area (mm²) rate (*) (sec) passes (sec) (m/sec) (°C.) mill 15625 1 13000 16.8 19.0 25 2.7 850 R 2 11000 15.4 17.5 25 2.7847 R 3 9500 13.6 16.3 25 2.7 847 R 4 8000 15.8 14.9 25 2.7 845 R 5 650018.8 13.5 25 2.7 844 R 6 5200 20.0 12.0 25 2.7 845 R 7 4150 20.2 10.8 252.7 843 R 8 3300 20.5 9.6 25 2.7 834 R 9 2450 25.8 8.3 25 2.7 830 R 101850 24.5 7.2 25 2.7 829 R 11 1450 21.6 6.4 5 0.350 821 T 12 1150 20.75.7 5 0.466 814 T 13 900 21.7 5.0 5 0.581 803 T 14 700 22.2 4.4 5 0.733794 T 15 550 21.4 3.9 5 0.871 790 T 16 420 23.6 3.4 5 0.982 782 T 17 32023.8 1.125 772 T

[0086] TABLE 21 Fatigue Microstructure (primary α) Forging strength 1/4DCenter section characteristics Rolling 0.2% Smooth Notch Grain VolumeGrain Volume Occur- Occurrence con- PS UTS E1 RA test test size fractionAspect size fraction Aspect rence of rough dition (MPa) (MPa) (%) (%)(Kt = 1) (Kt = 3) (μm) (%) ratio (μm) (%) ratio of crack surface RemarkB01 931 1030 20.4 51.9 565 230 2.5 66 1.5 2.7 66 1.8 Not Not E occurredoccurred B02 885 1009 3.5 12.3 350 120 3.7 59 4.1 — — — OccurredOccurred C B03 879 1010 4.1 13.5 355 125 3.4 58 4.4 — — — OccurredOccurred C B04 881 1011 4.1 11.6 365 115 — — — — — — Occurred Occurred CB05 874 1014 3.8 11.1 360 100 3.8 29 4.2 — — — Occurred Occurred C B06921 1020 20.0 50.8 560 225 5.4 60 2.1 5.8 68 2.2 Not Not E occurredoccurred B07 887 1005 3.7 12.1 355 120 5.9 31 4.3 — — — OccurredOccurred C B08 930 1030 20.5 52.3 570 240 1.7 67 1.9 1.9 69 2.3 Not NotE occurred occurred B09 929 1027 20.1 50.1 550 210 4.1 62 1.7 4.9 64 2.1Not Not E occurred occurred B10 911 1019 14.8 43.3 480 185 11.4 89 2.812.0 88 3.2 Not Occurred C occurred B11 863 1012 3.6 9.8 230 95 13.2 852.9 — — — Occurred Occurred C B12 902 1011 13.8 42.1 440 175 14.5 80 3.015.0 89 3.4 Not Occurred C occurred B13 899 987 12.1 38.2 395 155 5.5 854.2 5.8 87 4.5 Not Occurred C occurred B14 884 971 13.7 34.5 345 115 5.284 4.2 — — — Occurred Occurred C B15 894 955 11.9 33.3 340 120 5.3 814.3 — — — Occurred Occurred C B16 910 1014 17.4 40.1 505 205 6.2 63 2.56.4 60 2.7 Not Not E occurred occurred B17 914 1021 18.3 42.3 510 2055.8 64 2.7 6.3 61 2.9 Not Not E occurred occurred B18 902 1008 15.6 40.1500 200 6.5 60 3.1 6.6 60 3.3 Not Not E occurred occurred

What is claimed:
 1. An α+β type titanium alloy bar consistingessentially of 4 to 5% Al, 2.5 to 3.5% V, 1.5 to 2.5% Fe, 1.5 to 2.5%Mo, by mass, and balance of Ti, and having 10 to 90% of volume fractionof primary α phase, 10 μm or less of average grain size of the primary αphase, and 4 or less of aspect ratio of the grain of the primary α phaseon the cross sectional plane parallel in the rolling direction of thebar.
 2. The α+β type titanium alloy bar of claim 1, wherein the volumefraction of primary α phase is 50 to 80%, and the average grain size ofthe primary α phase is 6 μm or less.
 3. A method for manufacturing anα+β type titanium alloy bar comprising the step of hot rolling an α+βtype titanium alloy consisting essentially of 4 to 5% Al, 2.5 to 3.5% V,1.5 to 2.5% Fe, 1.5 to 2.5% Mo, by mass, and balance of Ti, whilekeeping the surface temperature thereof to β transus or below.
 4. Themethod for manufacturing an α+β type titanium alloy bar of claim 3comprising the steps of: heating an α+β type titanium alloy having a βtransus of Tβ ° C. while keeping the surface temperature thereof between(Tβ−150) and Tβ ° C.; and hot rolling the heated α+β type titanium alloywhile keeping the surface temperature thereof during hot rolling between(Tβ−300) and (Tβ−50) ° C. and keeping the finish surface temperaturethereof, as the surface temperature immediately after the final rollingpass, between (Tβ−300) and (Tβ−100) ° C.
 5. The method for manufacturingan α+β type titanium alloy bar of claim 4, wherein the α+β type titaniumalloy is hot rolled at a reduction rate of 40% or less per rolling pass.6. The method for manufacturing an α+β type titanium alloy bar of claim4, wherein the rolling speed is selected to 6 m/sec or less when areverse rolling mill is applied to hot rolling.
 7. The method formanufacturing an α+β type titanium alloy bar of claim 4, wherein therolling speed is selected to 1.5 m/sec or less when tandem rolling millsare applied to hot rolling.
 8. The method for manufacturing an α+β typetitanium alloy bar of claim 4, wherein when the α+β type titanium alloyhaving 3500 mm² or larger cross sectional area in normal to the rollingdirection is hot rolled to the cross sectional area of S mm², a waitingtime before starting succeeding rolling is 0.167×S^(1/2) or more sec. 9.The method for manufacturing an α+β type titanium alloy bar of claim 4,wherein the α+β type titanium alloy is reheated during hot rolling.